Increasingly, in recent years communications need is met by digital transmission. This is manifested in the form of delta modulation of input analog signal into a digital form and then the signal in the digital form is frequency modulated and transmitted. At the receiving end the frequency modulated received signal is frequency demodulated and the frequency demodulated signal is received in a proper manner. If the received signal is in the form of delta modulated signal, the delta modulated signal is then converted into analog signal by use of a digital to analog converter.
It is generally known to provide a receiver that can receive demodulated signal, that is, a demodulated signal in the analog form or in the digital form. If it is in the digital form, the digital signal is first converted into an analog signal by a digital to analog converter and then the analog signal is utilized to drive the output means which may be in the form of audio output.
Typically, the detector for detecting the delta modulated signal according to prior art includes means for detecting a predetermined code signal to sense the presence of an incoming delta modulated signal. Upon detection of such a signal, the incoming delta modulated signal is converted into analog signal and received.
Such a prior art detector is often susceptible to failure in that it is not capable of discriminating and detecting incoming data in the presence of high level of noise and tends to detect incoming noise signal as if it is a valid digital data. Various attempts have been made to overcome such a shortcoming and usually the attempts have been made in the form of rather elaborate filtering circuit arrangement to filter out the noise content of the incoming digital data for avoiding false detection. It has been found, however, that none of the prior art detectors designed to detect delta modulated signal in a noisy environment is completely satisfactory and require still further improvements.